4 INTRODUCTORY. 



furnished by the rocks showing that the Birds had a reptilian 

 origin ; but the Birds did not form an evolutionary stage 

 between the Reptile and the Mammal, but evolved side by side 

 with the latter. 



The existing British Mammals represent the six orders 

 nsectivora (shrews, mole, and hedgehog), Chiroptera (bats), 

 Carnivora (beasts of prey), Rodentia (gnawing animals), 

 Cetacea (whales and dolphins), and Ungulata (hoofed animals). 

 These all agree with the Reptiles and Batrachians in having a 

 many-jointed internal skeleton, a bony framework giving 

 support to a system of powerful muscles ; and of this frame- 

 work the most important feature is the long backbone or 

 vertebral column consisting of a number of bony rings jointed 

 together by outgrowths or " processes," and held in position by 

 strong ligaments. This attachment of the rings by their flat 

 surfaces produces the spine or vertebral column, with a canal 

 on its upper half in which lies the spinal cord. This column, 

 for descriptive purposes, is divided into regions cervical, 

 dorsal, lumbar, sacral, and caudal. The number of rings or 

 vertebras in each region varies somewhat in the different classes 

 and orders, but as a rule the cervical or neck vertebras are 

 seven ; the dorsal, to which the ribs are connected, are about 

 thirteen (extreme numbers are nine and twenty-two) ; the 

 vertebras of the lumbar or loin region are usually six or seven, 

 but they vary inversely to those of the dorsal from two to 

 twenty-three ; the sacral vertebras (about five) are in the adult 

 fused together into a solid bone (sacrum) of triangular shape ; 

 the caudal vertebras vary from three (man) to nearly fifty, 

 according to the length of tail common to the genus or species. 



In front of the neck is the skull, in the Mammals a bony case 

 containing the brain and organs of sense, made up of plates 

 interlocking by their zigzag margins ; in the Reptiles and lower 

 vertebrates a more or less open framework. The lower jaw, or 

 mandible, is in adult Mammals the only part of the skull that 



